Photoacoustic imaging apparatus and photoacoustic imaging method

ABSTRACT

A photoacoustic imaging apparatus performs imaging of an optical absorber. The photoacoustic imaging apparatus includes a light source, a detector configured to detect an acoustic wave generated from the optical absorber that has absorbed energy of light emitted from the light source, and a signal processing unit configured to form an image of the optical absorber. The signal processing unit stores information indicating whether a rate of change in pressure of the acoustic wave detected by the detector is positive or negative before performing a waveform process on the acoustic wave.

TECHNICAL FIELD

The present invention relates to an imaging apparatus using aphotoacoustic effect and to a photoacoustic imaging method.

BACKGROUND ART

In recent years, there has been suggested a method for obtaining adistribution of optical characteristic values in vivo at high resolutionby using a characteristic of ultrasound, which is less likely to scatterin vivo compared to light.

According to PTL, a living body is irradiated with pulsed lightgenerated by a light source, an acoustic wave that is generated frombiological tissue through energy absorption of the pulsed light isdetected, and a signal corresponding to the detected acoustic wave isanalyzed, whereby a distribution of optical characteristic values invivo is obtained. The imaging using an acoustic wave obtained throughirradiation of a living body with light is generally calledphotoacoustic imaging.

Regarding a photoacoustic imaging method, the following technique isknown. That is, when a photoacoustic wave generated by irradiating aspherical optical absorber 10 with light is detected by an acoustic wavedetector 20 as illustrated in FIG. 9A, N-shape acoustic pressureinformation illustrated in FIG. 9B is obtained if optical absorption ofthe optical absorber 10 is even (see NPL).

The value obtained by multiplying a time width of the N-shape waveformby sonic speed is a value in which the size of the optical absorber 10(here, the diameter of the sphere) is reflected. Also, the time when theN-shape waveform is detected reflects position information of theoptical absorber 10. Furthermore, in a case where the intensity of lightthat arrives at the optical absorber 10 is equal, the magnitude of asignal having the N-shape waveform is proportional to the absorptioncoefficient of the optical absorber 10.

As described above, in photoacoustic imaging, an image of the opticalabsorber 10 is reconstructed by using data obtained from a photoacousticwave.

In the above-described photoacoustic imaging performed by detectingultrasound that is generated through optical absorption, a tissue havingan optical absorption coefficient higher than that of a medium around anoptical absorber is imaged. For example, a blood vessel in a living bodyhas an optical absorption coefficient higher than that of a surroundingmedium. Imaging of a blood vessel has been studied.

As a method for processing a photoacoustic wave signal obtained from adetector, a waveform process such as envelope detection can be used. Byperforming an image formation process after the waveform process, an invivo optical characteristic distribution can be imaged.

When photoacoustic wave signals measured by detectors provided atvarious positions are used, an in vivo optical characteristicdistribution can be imaged by using image reconstruction using a methodinvolving an aperture synthesis process, such as delay and sum, and awaveform process, such as envelope detection.

In the near-infrared region of about 700 to 1100 nm that is used inphotoacoustic imaging, there is a tissue having an optical absorptioncoefficient higher than that of a surrounding tissue, such as a bloodvessel, and also there is a tissue having an optical absorptioncoefficient lower than that of a surrounding tissue, such as a calcifiedsubstance.

For this reason, in an image formation method simply using envelopedetection described above, the difference in optical absorptioncoefficient between a subject and a surrounding medium can be detectedonly in the form of an absolute value. Accordingly, it has beendifficult to determine whether the optical absorption coefficient of thesubject is higher or lower than that of the surrounding medium.

Citation List Patent Literature

-   PTL 1: U.S. Pat. No. 5,713,356

Non Patent Literature

-   NPL 1: L. V. Wang, et al. “Biomedical Optics-Principles and    Imaging”, Wiley, Ch. 12, 2007

SUMMARY OF INVENTION

The present invention provides a photoacoustic imaging apparatus and aphotoacoustic imaging method that can perform imaging by distinguishinga tissue having an optical absorption coefficient lower than that of asurrounding medium and a tissue having an optical absorption coefficienthigher than that of a surrounding medium from each other.

A photoacoustic imaging apparatus according to an aspect of the presentinvention includes a light source, a detector configured to detect anacoustic wave generated from an optical absorber that has absorbedenergy of light emitted from the light source, and a signal processingunit configured to form an image of the optical absorber by storinginformation indicating whether a rate of change in pressure of theacoustic wave detected by the detector is positive or negative beforeperforming a waveform process on the acoustic wave.

A photoacoustic imaging method according to an aspect of the presentinvention includes a step of emitting light from a light source, a stepof detecting an acoustic wave generated from an optical absorber thathas absorbed energy of the light emitted from the light source, and astep of storing information indicating whether a rate of change inpressure of the detected acoustic wave is positive or negative beforeperforming a waveform process on the acoustic wave.

According to the aspects of present invention, a photoacoustic imagingapparatus and a photoacoustic imaging method that can perform imaging bydistinguishing a tissue having an optical absorption coefficient lowerthan that of a surrounding medium and a tissue having an opticalabsorption coefficient higher than that of a surrounding medium fromeach other can be provided.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1]FIG. 1 illustrates an exemplary configuration of a photoacousticimaging apparatus according to an embodiment of the present invention.

[FIG. 2A]FIG. 2A is a diagram for explaining an imaging method for anoptical absorber having an optical absorption coefficient higher thanthat of a background.

[FIG. 2B]FIG. 2B is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient higher thanthat of a background.

[FIG. 2C]FIG. 2C is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient higher thanthat of a background.

[FIG. 2D]FIG. 2D is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient higher thanthat of a background.

[FIG. 3A]FIG. 3A is a diagram for explaining an imaging method for anoptical absorber having an optical absorption coefficient lower thanthat of a background.

[FIG. 3B]FIG. 3B is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient lower thanthat of a background.

[FIG. 3C]FIG. 3C is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient lower thanthat of a background.

[FIG. 3D]FIG. 3D is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient lower thanthat of a background.

[FIG. 3E]FIG. 3E is a diagram for explaining the imaging method for anoptical absorber having an optical absorption coefficient lower thanthat of a background.

[FIG. 4]FIG. 4 is a flowchart according to a first embodiment.

[FIG. 5]FIG. 5 is a flowchart according to a second embodiment.

[FIG. 6]FIG. 6 is a flowchart according to a third embodiment.

[FIG. 7A]FIG. 7A is a diagram for explaining a simulation modelaccording to an example of the present invention.

[FIG. 7B]FIG. 7B is a diagram for explaining the simulation modelaccording to the example of the present invention.

[FIG. 8A]FIG. 8A is a diagram for explaining a simulation resultaccording to the example of the present invention.

[FIG. 8B]FIG. 8B is a diagram for explaining the simulation resultaccording to the example of the present invention.

[FIG. 9A]FIG. 9A is a diagram for explaining a photoacoustic signalhaving an N-shape waveform.

[FIG. 9B]FIG. 9B is a diagram for explaining the photoacoustic signalhaving an N-shape waveform.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described in detail inaccordance with the accompanying drawings.

FIGS. 2A to 2D are diagrams for explaining an image processing method ina case where the optical absorption coefficient of a spherical tissue210 is higher than that of a surrounding medium 220. As illustrated inFIG. 2A, when a photoacoustic wave that is generated by irradiating thetissue 210 with pulsed light 230 is detected by a detector 240, waveformdata 250 illustrated in FIG. 2B is obtained. When light is evenlyabsorbed by an absorber, the waveform data is N shaped.

The method for processing that is performed thereafter has twoalternatives.

In one of them, a process based on envelope detection is performed onthe waveform data illustrated in FIG. 2B, as illustrated in FIG. 2C. Animage formation process is performed on the basis of the processedwaveform data, and an image 280 of the optical absorber reflecting theoptical absorption coefficient is displayed on a display unit 270 asillustrated in FIG. 2D.

In the other method, pieces of waveform data detected at variouspositions are added in an image reconstruction method using an algorithmof a synthetic aperture process or the like, envelope detection is thenperformed, and the image 280 of the optical absorber reflecting theoptical absorption coefficient is displayed on the display unit 270.

On the other hand, FIGS. 3A to 3E are diagrams for explaining an imageprocessing method in a case where the optical absorption coefficient ofa spherical tissue 310 is lower than that of the surrounding medium 220.

The inventers of the present invention have found through study that, insuch a case where the optical absorption coefficient of the tissue 310is lower than that of the surrounding medium 220, data 350 having anN-shape waveform in which positive/negative is inverted with respect tothe waveform illustrated in FIG. 2B (inverted N-shape waveform) isdetected, as illustrated in FIG. 3B.

That is, in a case where the optical absorption coefficient of a targetportion is higher than that of a surrounding tissue, a photoacousticwave signal rises at a signal start point (i.e., the rate of change inpressure is positive at the start). On the other hand, in a case wherethe optical absorption coefficient of a target portion is lower thanthat of a surrounding tissue, a photoacoustic wave signal falls at asignal start point (i.e., the rate of change in pressure is negative atthe start).

The present invention has been made using the above-described findings,and is characterized in that imaging is performed with a tissue havingan optical absorption coefficient lower than that of a surroundingmedium being distinguished from a tissue having an optical absorptioncoefficient higher than that of a surrounding medium.

FIRST EMBODIMENT

FIG. 1 illustrates an exemplary configuration of a photoacoustic imagingapparatus 100 according to a first embodiment of the present invention.

The photoacoustic imaging apparatus 100 according to this embodiment iscapable of imaging optical characteristic values in vivo and aconcentration distribution of substances constituting biological tissueobtained from information about those values for the purpose ofdiagnosing a malignant tumor, a blood vessel disease, and the like, orobserving a progress of chemical treatment. Particularly, thephotoacoustic imaging apparatus 100 is capable of imaging an opticalabsorber having an optical absorption coefficient lower than that of asurrounding medium.

In the photoacoustic imaging apparatus 100 according to this embodiment,light emitted from a light source 130 propagates through an opticalfiber 140, so that a living body 120 is irradiated with the light. Theliving body 120 as a surrounding medium has an optical absorber 110.Light energy absorbed by the optical absorber 110 is converted into anacoustic wave 160. The acoustic wave 160 is detected by detectors 150.The optical absorber 110 generates an acoustic wave in accordance with adifference in optical absorption coefficient between the opticalabsorber 110 and a surrounding medium even in a case where the opticalabsorption coefficient of the optical absorber 110 is lower than that ofa surrounding medium constituting a subject.

Also, the photoacoustic imaging apparatus 100 according to thisembodiment includes a signal processing unit 170 that obtainsdistribution information about optical characteristic values by usingelectric signals obtained from the detectors 150.

The light source 130 generates pulsed light. The pulsed light is in anorder of several nanoseconds to several hundreds of nanoseconds, and thewavelength thereof should be 700 nm or more and 1100 nm or less. A laseris used as the light source 130, but a light-emitting diode or the likecan be used in place of the laser. When a dye laser or an OPO (OpticalParametric Oscillator) capable of converting an oscillation wavelengthis used, a difference in distribution of optical characteristic valuesdue to the wavelength can be measured.

Each of the detectors 150 absorbs energy of light with which the livingbody 120 is irradiated, detects an acoustic wave that is generated fromthe optical absorber 110 in the living body 120 in accordance with adifference in optical absorption coefficient, and converts the acousticwave into an electric signal. As the detector 150, a focus transducercapable of receiving only an acoustic wave generated from a specificregion can be used. When the focus transducer is used, the position ofan optical absorber can be specified, so that a process of imagereconstruction, such as delay and sum, is not necessarily performed.Also, any type of acoustic wave detector may be used as the detector 150as long as the detector is capable of detecting an acoustic wave signal,for example, a transducer using a piezoelectric phenomenon, a transducerusing optical resonance, or a transducer using a change in capacitance.In this embodiment, the plurality of detectors 150 are placed on asurface of the living body 120. Alternatively, a single detector may beused to scan the surface of the living body 120.

The signal processing unit 170 analyzes the electric signal, wherebydistribution information about optical characteristic values of theliving body 120 is obtained. As the signal processing unit 170, anydevice may be used as long as the intensity and time change of anacoustic wave can be stored and converted into data of a distribution ofoptical characteristic values by using a computing unit. For example, anoscilloscope and a computer capable of analyzing data stored in theoscilloscope can be used.

In a case of the spherical tissue 210 illustrated in FIG. 2A, aphotoacoustic signal generated from the tissue 210 having an opticalabsorption coefficient higher than that of the surrounding medium 220rises at the start point as illustrated in FIG. 2B (N-shape waveform).

On the other hand, in a case of the spherical tissue 310 illustrated inFIG. 3A having an optical absorption coefficient lower than that of thesurrounding medium 220, the signal falls at the start point asillustrated in FIG. 3B (inverted N-shape waveform).

A description will be given with reference to the flowchart in FIG. 4about a signal processing method for imaging a tissue having an opticalabsorption coefficient lower than that of a surrounding medium on thebasis of the above-described findings.

First, a focus transducer serving as the detector 150 detects aphotoacoustic signal (S410).

Subsequently, the signal processing unit 170 stores informationindicating whether the rate of change in pressure of the photoacousticsignal has a positive value or a negative value (S420). In order toadapt the data of the photoacoustic signal to an absorptioncharacteristic of an optical absorber, an envelope detection process(e.g., absolute value is obtained after Hilbert transform) is performedas a waveform process (S430). Examples of acoustic pressure informationobtained through the envelope detection process are illustrated in FIGS.2C and 3C.

Subsequently, it is determined whether the photoacoustic signal rises atthe signal start point in change in pressure of the photoacoustic signal(S440). Depending on the characteristic of the detector used, correctionshould be performed before determination in accordance with a frequencyresponse characteristic of the detector. That is, deconvolution shouldbe performed on the received photoacoustic signal in accordance with thefrequency response characteristic of the detector used.

Here, if it is determined in S440 that the photoacoustic signal rises,image formation is performed in the same manner as in a conventionalphotoacoustic imaging apparatus (S460). On the other hand, if it isdetermined in step S440 that the photoacoustic signal falls, thepositive/negative of acoustic pressure data after envelope detection isinverted (S450). With this process in S450, the waveform data 360illustrated in FIG. 3D can be obtained. Subsequently, image formation isperformed in the same manner as in the case where the signal rises(S460). In order to perform image formation, time data of the waveformdata 360 may be converted into position data and then be plotted.

Finally, the signal of an image obtained in S460 is output from thesignal processing unit 170, whereby the image of the optical absorber isdisplayed on the image display units 180 and 270. In this case, asillustrated in FIGS. 2D and 3E, the image 280 of the tissue having anoptical absorption coefficient higher than that of the surroundingtissue and the image 380 of the tissue having an optical absorptioncoefficient lower than that of the surrounding tissue can be displayedon the image display unit 270 in different display forms. In order toexpress different types of tissues having different optical absorptioncoefficients, different display colors or different color tones can beused (e.g., gradation is changed). Alternatively, an identificationsymbol (e.g., + or −) may be given to a tissue having an opticalabsorption coefficient higher or lower than that of a surroundingtissue.

The use of the photoacoustic imaging apparatus 100 according to thefirst embodiment enables imaging of not only a tissue having an opticalabsorption coefficient higher than that of a surrounding medium in vivobut also a tissue having an optical absorption coefficient lower thanthat of a surrounding medium even when envelope detection is used.

It is known that the optical absorption coefficient of a substancevaries in accordance with the wavelength of light. In a case where lightof a plurality of wavelengths is used, optical absorption coefficientsin vivo for the respective wavelengths are calculated. Then, thosecoefficients are compared with the wavelength dependency unique tosubstances constituting biological tissue (glucose, collagen,oxidation-reduction hemoglobin, etc.), so that a concentrationdistribution of the substances constituting a living body can be imaged.

Furthermore, in a case where measurement is performed with a pluralityof wavelengths by using the photoacoustic imaging apparatus 100according to this embodiment, a high/low relationship between theoptical absorption coefficient of a substance and that of a surroundingmedium can be determined, so that the substance as a measurement targetcan be specified.

Now, a description will be given about an example of determining theexistence of substances A, B, and C, the optical absorption coefficientthereof being different from that of a surrounding medium. Assume thatthe optical absorption coefficient of the substance A is higher thanthat of a surrounding medium in both of certain two wavelengths lambda-1and lambda-2. Also, assume that the optical absorption coefficient ofthe substance B is higher in the wavelength lambda-1 and is lower in thewavelength lambda-2. Also, assume that the optical absorptioncoefficient of the substance C is lower in both of the wavelengthslambda-1 and lambda-2.

In this case, when measurement is performed by using the two wavelengthslambda-1 and lambda-2, a positive signal (a signal that rises at thesignal start point) is output from the substance A in the bothwavelengths. As for the substance B, a positive signal is obtained whenthe wavelength lambda-1 is used, whereas a negative signal (a signalthat falls at the signal start point) is output when the wavelengthlambda-2 is used. As for the substance C, a negative signal is output inthe both wavelengths. By comparing these results, the substances A, B,and C can be distinguished from each other.

As described above, by using the photoacoustic imaging apparatusincluding the light source capable of emitting light of a plurality ofwavelengths and the signal processing unit, the substances A, B, and Ccan be identified or a distribution of the substances A, B, and C can beeasily determined on the basis of a positive/negative value of an imagereconstruction result.

That is, the comparison between the shape of a first acoustic waveobtained through irradiation with light of the wavelength lambda-1 andthe shape of a second acoustic wave obtained through irradiation withlight of the wavelength lambda-2 enables analysis of a substance. Such aprocess can be performed by the above-described signal processing unit170 or another unit.

SECOND EMBODIMENT

A photoacoustic imaging apparatus according to a second embodiment isdifferent from the photoacoustic imaging apparatus 100 according to thefirst embodiment in that an image formation process is performed after anormal envelope detection process has been performed. That is, in thefirst embodiment, image formation is performed after thepositive/negative of acoustic pressure data after envelope detection hasbeen inverted. On the other hand, in the second embodiment, imageformation is performed without performing such a process. Theconfiguration of the apparatus according to the second embodiment is thesame as that of the first embodiment except for signal processing andimage reconstruction, and thus the description thereof is omitted.

FIG. 5 is a flowchart of signal processing and image reconstructionaccording to the second embodiment.

First, a focus transducer serving as the detector 150 detects aphotoacoustic signal (S510).

Subsequently, the signal processing unit 170 stores pressure changeinformation of the photoacoustic signal (S520). After an envelopedetection process has been performed to adapt the data of thephotoacoustic signal to an absorption characteristic of an opticalabsorber (S530), an. image formation process is performed (S540). In theimage formation process, time data of the waveform data 360 is convertedinto position data and is plotted.

In the image formation process according to the first embodiment, imagereconstruction is performed after the positive/negative of the dataobtained through envelope detection has been inverted when a fall at thesignal start point is detected. However, according to the secondembodiment, the image formation process is performed without inversionof the positive/negative of data obtained through envelope detection.

Subsequently, it is determined whether the photoacoustic signal falls onthe basis of the pressure change information of the photoacoustic signalstored in S520 (S550). Depending on the characteristic of the detectorused, correction should be performed in accordance with the frequencyresponse characteristic of the detector before determination.

If it is determined in S550 that the photoacoustic signal rises, theimage obtained in S540 is displayed on the image display units 180 and270 (S570).

On the other hand, if it is determined in S550 that the photoacousticsignal falls, information indicating that the optical absorptioncoefficient of the tissue is lower than that of a background is added(S560). After that, the signal of an image added with the information isoutput from the signal processing unit 170 and the image is displayed onthe image display units 180 and 270 (S570).

Accordingly, an image of a tissue having an optical absorptioncoefficient higher than that of a surrounding tissue and an image of atissue having an optical absorption coefficient lower than that of asurrounding tissue can be displayed in different display forms. In orderto express different types of tissues having different opticalabsorption coefficients, different display colors or different colortones can be used. Alternatively, an identification symbol may be givento a tissue having an optical absorption coefficient lower than that ofa surrounding tissue.

In the photoacoustic imaging apparatus according to the secondembodiment, as in the apparatus according to the first embodiment, asubstance as a measurement target can be specified by using light of aplurality of wavelengths.

THIRD EMBODIMENT

A photoacoustic imaging apparatus according to a third embodiment isdifferent from the photoacoustic imaging apparatuses according to thefirst and second embodiments in that image reconstruction is performedby using an algorithm based on delay and sum or an algorithm based onFourier transform. As a detector according to this embodiment, adetector capable of detecting signals from various regions should beused. Other than the foregoing points, the configuration of theapparatus according to the third embodiment is the same as thataccording to the first embodiment, and thus the description thereof isomitted.

FIG. 6 is a flowchart of signal processing and image reconstructionaccording to the third embodiment.

First, the detector 150 detects a photoacoustic signal (S610).

Subsequently, the signal processing unit 170 stores pressure changeinformation of the photoacoustic signal (S620), performs Hilberttransform on data of the photoacoustic signal (S630), and performs imagereconstruction (S640). As a method for the image reconstruction, analgorithm based on delay and sum or an algorithm based on Fouriertransform is used. After that, an absolute value of the obtained imagedata is obtained (S650).

Subsequently, it is determined whether the photoacoustic signal falls onthe basis of the pressure change information of the photoacoustic signalstored in S620 (S660). Depending on the characteristic of the detectorused, correction should be performed in accordance with the frequencyresponse characteristic of the detector before determination.

If it is determined in S660 that the photoacoustic signal rises, theobtained signal of an image is output from the signal processing unit170 and the image is displayed on the image display units 180 and 270(S680).

On the other hand, if it is determined in S660 that the photoacousticsignal falls, information indicating that the optical absorptioncoefficient of the tissue is lower than that of a background is added(S670). After that, the signal of an image added with the information isoutput from the signal processing unit 170 and the image is displayed onthe image display units 180 and 270 (S680).

Accordingly, an image of a tissue having an optical absorptioncoefficient higher than that of a surrounding tissue and an image of atissue having an optical absorption coefficient lower than that of asurrounding tissue can be displayed in different display forms. In orderto express different types of tissues having different opticalabsorption coefficients, different display colors or different colortones can be used. Alternatively, an identification symbol (e.g., + or−) may be given.

Alternatively, the data obtained through image reconstruction afterenvelope detection may be displayed on the image display units 180 and270 by inverting the positive/negative of the optical absorptioncoefficient of the optical absorber.

In the photoacoustic imaging apparatus according to the thirdembodiment, as in the apparatus according to the first embodiment, asubstance as a measurement target can be specified by using light of aplurality of wavelengths.

EXAMPLE 1

Simulation was performed for a photoacoustic signal obtained in thefollowing cases: a case where the optical absorption coefficient of ameasurement target is higher than that of a surrounding tissueconstituting a subject; and a case where the optical absorptioncoefficient of a measurement target is lower than that of a surroundingtissue constituting a subject.

A simulation model is illustrated in FIGS. 7A and 7B. Specifically, atwo-dimensional space was set and simulation was performed for aphotoacoustic signal that is obtained at a point 640 when irradiationwith light 630 from the bottom of the figure was performed. An opticalabsorber 610 as a measurement target, which is a circle having adiameter of 1 cm, was positioned at 1 cm from a light irradiation point.The sonic speed was 1500 m/s.

The optical absorption coefficient of a background region 620 was 0.1cm⁻¹, and the equivalent scattering coefficient thereof was 10 cm⁻¹. Theoptical absorption coefficient of the optical absorber 610 was 1.0 cm⁻¹,and the equivalent scattering coefficient thereof was 10 cm ⁻¹. Theoptical absorption coefficient of an optical absorber 710 was 0 cm⁻¹,and the equivalent scattering coefficient thereof was 10 cm⁻¹.

A simulation result about a time-lapse change of acoustic pressure atthe point 640 is illustrated in FIGS. 8A and 8B. In the case of theoptical absorber 610 having an optical absorption coefficient higherthan that of the region 620, a positive photoacoustic signal in which asignal from a measurement target rises at the start point was obtained(FIG. 8A). On the other hand, in the case of the optical absorber 710having an optical absorption coefficient lower than that of the region620, a negative photoacoustic signal in which a signal from ameasurement target falls at the start point was obtained (FIG. 8B).

Accordingly, it is understood that a positive photoacoustic signal thatrises at the start point is generated when the optical absorptioncoefficient is higher than that of a surrounding region and that anegative photoacoustic signal that falls at the start point is generatedwhen the optical absorption coefficient is lower than that of asurrounding region.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-316042, filed Dec. 11, 2008, which is hereby incorporated byreference herein in its entirety.

1. A photoacoustic imaging apparatus comprising: a light source; adetector configured to detect an acoustic wave generated from an opticalabsorber that has absorbed energy of light emitted from the lightsource; and a signal processing unit configured to form a signal of animage of the optical absorber, wherein the signal processing unit storesinformation indicating whether a rate of change in pressure of theacoustic wave detected by the detector is positive or negative beforeperforming a waveform process on the acoustic wave, determines whetherthe rate of change in pressure of the acoustic wave starts from positiveor negative, and forms the signal of the image using a result of thedetermination.
 2. The photoacoustic imaging apparatus according to claim1, wherein the waveform process is envelope detection.
 3. Thephotoacoustic imaging apparatus according to claim 1, wherein, in a casewhere the rate of change in pressure of the acoustic wave is negative atthe start, the signal processing unit outputs a signal for displayinginformation indicating that the optical absorber has an opticalabsorption coefficient lower than an optical absorption coefficient of asurrounding region of the optical absorber.
 4. The photoacoustic imagingapparatus according to claim 1, wherein, in a case where the rate ofchange in pressure of the acoustic wave is negative at the start, thesignal processing unit converts acoustic pressure information obtainedthrough the waveform process from a positive value to a negative valueor vice versa, and forms a signal of an image of the optical absorber onthe basis of the converted acoustic pressure information.
 5. Thephotoacoustic imaging apparatus according to claim 1, wherein thedetector is a focus transducer.
 6. The photoacoustic imaging apparatusaccording to claim 3, further comprising an image display unit, whereinthe image display unit displays an image of an optical absorber havingan optical absorption coefficient lower than an optical absorptioncoefficient of a surrounding region by using a display color differentfrom a display color of the surrounding region or by using a color tonedifferent from a color tone of the surrounding region.
 7. Thephotoacoustic imaging apparatus according to claim 3, further comprisingan image display unit, wherein the image display unit displays an imageof a tissue having an optical absorption coefficient lower than anoptical absorption coefficient of a surrounding region, anidentification symbol being added to the image.
 8. The photoacousticimaging apparatus according to claim 1, wherein the signal processingunit performs correction in accordance with a frequency response of thedetector on the acoustic wave detected by the detector before storingthe information indicating whether the rate of change in pressure of theacoustic wave is positive or negative.
 9. The photoacoustic imagingapparatus according to claim 1, wherein the light source is capable ofemitting light having a first wavelength and light having a secondwavelength that is different from the first wavelength, and wherein thesignal processing unit identifies the optical absorber by comparing ashape of a first acoustic wave obtained through irradiation with thelight having the first wavelength with a shape of a second acoustic waveobtained through irradiation with the light having the secondwavelength.
 10. A photoacoustic imaging method comprising: a step ofemitting light from a light source; a step of detecting an acoustic wavegenerated from an optical absorber that has absorbed energy of the lightemitted from the light source; a step of storing information indicatingwhether a rate of change in pressure of the detected acoustic wave ispositive or negative before performing a waveform process on theacoustic wave; a step of determining whether the rate of change inpressure of the acoustic wave starts from positive or negative; and astep of forming a signal of an image using a result of the determiningstep.